We estimate the effects of interstate highways on the growth of US cities between 1983 and 2003. We find that a 10% increase in a city's initial stock of highways causes about a 1.5% increase in its employment over this 20 year period. To estimate a structural model of urban growth and transportation, we rely on an instrumental variables estimation which uses a 1947 plan of the interstate highway system, an 1898 map of railroads, and maps of the early explorations of the US as instruments for 1983 highways.